1. Field of the Invention
The present invention generally relates to electrical circuits for power control, and more particularly to an electrically isolated circuit which is used as a driver for a power MOSFET operating in a pulse mode. The invention has particular applicability in heart defibrillators.
2. Description of the Prior Art
Many electrical devices require electrical power to be delivered in short bursts, or pulses. One such device is a defibrillator which is an electronic instrument used for stopping fibrillation during a heart attack by applying controlled electric pulses to the heart muscles. These devices typically use mechanical relays to switch current to the patient, but newer defibrillators use solid state switching circuits which include a power transistor, such as a metal-oxide semiconducting, field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT). In this type of circuit, the power transistor must be held off under normal conditions, and is turned on for only a very short duration to deliver energy to the patient.
A conventional circuit 10 used to drive a power transistor is shown in FIG. 2. In this circuit, an enhancement mode transistor 12 (MOSFET) is used to control the power transistor 14 (either a power MOSFET or IGBT). Transistor 12 is normally off, i.e., it does not conduct across its drain and source leads in its unenergized state. In order to turn transistor 12 on, and thereby hold power transistor 14 in its off state, a positive charge must be applied to the gate of transistor 12. This is accomplished when input transistor 16 turns off, charging capacitor 18 via a low voltage (e.g., 12 volt) power source and resistor 20. Secondary current is induced in a transformer 22 due to the magnetizing energy stored in the transformer, and flows from another capacitor 24 through a rectifier 26, and thence to the secondary windings of transformer 22 and back to capacitor 24. When transistor 16 is subsequently turned on, the voltage across capacitor 18 is applied across the primary winding of transformer 22. Secondary current will flow from capacitor 24 and another rectifier 28 to the secondary windings of transformer 22, and through a zener diode 30 and rectifier 32, thence to transistor 12 and back to capacitor 24. Capacitor 24 will discharge, turning transistor 12 off and allowing the gate of transistor 14 to charge and turn transistor 14 on, thereby powering the load 36. The charge on the gate of transistor 14 is limited by zener diode 34.
The charge applied to the gate of transistor 12 bleeds off fairly quickly due to leakage currents. Accordingly, this circuit is acceptable for use as a high frequency switcher, say in the range of 1 kHz to 100 kHz, which allows capacitor 24 to be charged and discharged on each pulse, and it provides electrical isolation across the transformer. This circuit is not, however, suitable for use with a defibrillator since a typical pulse may last as long as 25 ms. Also, in this prior art circuit, the resting state of the enhancement mode transistor 12 is off, meaning the gate of the power transistor 14 is at a high impedance state. In this state, any sort of noise on the gate (such as an electrostatic discharge event) would tend to turn the power transistor on. It would, therefore, be desirable and advantageous to devise a driver for a power transistor which could be configured as a high side driver and provide electrical isolation, but is further suitable for use with power-on pulses as long as or larger than 25 ms and also uses a driver whose default (resting) state holds the power transistor off, so that it is less likely to be triggered in a noisy environment.